Stealth and detection in M&M suffers significantly from being entirely binary past the lowest levels of capability (that is, mundane stealth and detection). I don't mean binary in the sense of being pass/fail; I mean there's never really a situation where chance or eventual detection comes into it. Once powers start getting involved, you either automatically detect the subject (which with trivial effort is what will always happen for detecting someone's presence), or automatically fail to detect it (which with trivial effort is what will always happen for detecting someone's powers), without regard to Stealth and Perception skills. This results in those skills being greatly devalued for characters who wish to accomplish serious scouting.

This is all exasperated by the fact that several of these options are very low-cost. For 2 PP, one of your powers (or arrays) can be flat-impossible to detect by any means. For 1 PP, you can auto-detect anyone in normal sensory range who is neither separated by walls nor intangible (add Ranged to your already Accurate, Radius Touch Sense; the Concealment power is prohibited from hiding you from touch).

And then you get into the 2 PP ability to see through all illusions, while Illusion can easily be one of the most expensive powers in the game if you want to affect all the senses, which is just sad.

I...also never quite got why M&M developed this beautiful ranks and measures system for easily scaling up real world values...and then stuck to the ludicrous D&D -1 per 10' Perception penalties.

Detection DC:

The base DC to detect something is 10. If a character is actively attempting to keep the subject hidden, it may either add its Stealth skill directly to the DC, or it may roll a Stealth check to set the DC rather than using the flat DC. Generally speaking, hiding something other than itself or on itself is a standard action, and the benefits end of the hidden subject moves from the spot. Hiding oneself, or hiding an object (or small creature) on your person is a move action and you can retain stealth even while moving. Someone not actively attempting to hide doesn't add Stealth. In the case of a Stealth check, the result generally lasts for the current scene, even if the character reveals itself and attempts to hide again.

This DC is then modified as follows:

Subject has Total Concealment from all of your senses: DC +20.
Subject has Total Concealment from all of your senses, except mundane Touch: DC +15.
Subject has Total Concealment from all of your Accurate and Acute senses: DC +10.
Subject has Total Concealment from all of your Accurate senses: DC +5.
Subject has at least Partial Concealment from all of your Accurate and Acute senses: DC +0.
Subject has at least Partial Concealment from all of your Accurate senses: DC -5.
Subject has no Concealment from at least one of your Accurate senses: DC -30.

Distance and size also matters. The above DC is appropriate out to 30' (Distance Rank 0). Each additional Distance Rank imposes a -4 penalty to the Perception check. Extended senses, as always, multiply the range between penalties by 10, so if you have Extended Vision 1, you take -0 out to 300', -4 out to 600', -8 out to 1,200', and so on.

Generally speaking, size adjusts Stealth thanks to how Growth and Shrinking work, but the modifier also applies even if Stealth isn't being used. Each Size Rank of the subject larger than -2 lowers the DC by 4, while each Size Rank smaller raises it by 4. Since this is now built into the detection system, these modifiers do ignore PL limits.

The size effect also applies to other senses. For example, a human walking at normal speed or speaking in conversational tones would be the "default size" for sound, while hushed tones might increase the DC by 4, a whisper by 8, and speaking "under one's breath" by 12. Likewise, speaking loudly would lower the DC to hear by 4, calling out by 8, and shouting or screaming by 12.

Detection Results:

A successful Perception check means you detect the subject. That is, you are aware of its presence. The details you discern depend on your degrees of success, as follows:

One Degree: You can recognize, generally, what the subject is. So you would know you're looking at a human or a dog, or that you're hearing a song or an argument, or that you're sensing a strong, dark emotion. If you detect it with an Accurate sense, you also know its position, allowing you to target it and to affect it with Perception range effects.

Two Degrees: If you detect it with an Acute sense, you get broad details. You can't identify the target, but you get a vague description or gist. For example, you'd know you're looking at a dark-haired male wearing blue, or hearing an argument about the quality of food, or sensing anger. This level of detail is not possible if your total check bonus is -20 or less (typically due to extreme distance).

Three Degrees: If your detect it with an Acute sense, you get specific details. You can identify the target (and probably recognize it, if you're familiar enough with it). For example, you'd get a clear look at the person's face and be able to recognize him, or hear the exact argument and be able to recognize the voices, or sense who the anger is directed towards and what is causing it. If you are using an Analytical sense, you get explicit, thorough details as appropriate to your sense. This level of detail is not possible if the subject has Total Concealment from the relevant sense, even if you beat the DC by this amount. This level of detail is not possible if your total check bonus is -8 or less (typically due to significant distance or circumstance penalties).

Once a character has been detected, it cannot hide again without a distraction or unless circumstances improve. Powers that might offer the opportunity (such as Teleport) should be treated as Distraction attempts, though the character may substitute its power rank for its Deception skill when using powers in this way, if it wishes, up to a maximum rank equal to its PL + 10.

A failed Perception check means you do not detect the subject. If the subject hasn't done anything to reveal its presence (most actions that manipulate objects or target other characters), you are unaware of it entirely, and if it attacks you, you are considered Surprised. Otherwise, you are aware of it by its actions, but can't pinpoint or identify it, which generally means you can't target it with actions (although a good guess or big enough area attack could still affect it). Since you can't pinpoint the target, all of its attacks against you are considered Surprise Attacks until you manage to detect it.

Note that even if you detect a target and perceive details, you still take any relevant penalties on attacks against the target due to Concealment.

When to Detect:

By default, detection is resolved as a Routine Check against the relevant DC. You are constantly detecting everything you could potentially perceive with a check result equal to 10 + your Perception rank. Perception is considered the active check, so a tied result is a one-degree success on detection.

You can also roll Perception checks actively in certain situations:

If you know what you are looking for, you can roll a Perception check against that specific subject as a move action. This is called a Scan action. You don't have to perfectly identify what you're looking for, but you have to be reasonably precise. So, "traps" or "enemies" is too broad, but "whoever just shot at me", "whatever made that noise", "whoever just opened the door", or a specific individual, for example, are all okay. Scanning covers your full range with that sense.

You can also Search a specific area as a standard action. You can Search an area about six feet (Distance Rank -2) on a side (or a character with a Size Rank of -2 or less) with a single action. Each application of Quickness to this action increases the Distance Rank by 1. Each rank of Rapid on the sense is equivalent to three ranks of Quickness (Limited 2 [One Task - Perception]), so each application of Rapid increases the Distance Rank by 3. When searching, you detect anything hidden or concealed in the area that you roll high enough to detect.

Any time a character targets another character with an action, everyone nearby can attempt to detect that character actively. Unless the action itself has specific concealment (through the Subtle and Insidious Extras), any senses that the character has Total Concealment from, it is considered to only have Partial Concealment from, for purposes of this check. Additionally, the character must roll a new Stealth check, which applies only to this specific check. It is, thus, very difficult to maintain complete stealth during active combat.

Powers and Detection:

The Concealment power, as normal, provides Total Concealment for the listed senses. Note, however, that due to the new rules for calculating detection DCs, Total Concealment does not necessarily mean immunity to detection. Even a character concealed from all senses could be detected from certain tells in the environment or even pure instinct.

The Subtle Extra removes a power's sensory signature from the normal senses, save touch (that is, Vision, Hearing, Smell, and Taste). The power itself is considered to have Partial Concealment from those senses, but no Concealment from other senses that could detect it (such as appropriate types of Awareness, or Infravision for a heat-based power, etc). This generally means the DC to detect a Subtle power with normal senses is 10 + the user's Stealth rank.

Two applications of the Subtle Extra removes a power's sensory signature from all senses but touch. The power is considered to have Total Concealment from mundane senses except touch (Vision, Hearing, Smell, and Taste), and Partial Concealment from super-senses. Resultantly, the DC to detect the power is generally 25 + Stealth for mundane senses, or 10 + Stealth for super senses.

Subtle powers used by a character with Total Concealment from relevant senses do not downgrade the character's Concealment to Partial for purposes of reactive checks to detect the character, unless the power successfully causes an effect. Even in this case, the downgraded Concealment only applies to characters who were successfully affected. This benefit also applies to Indirect powers with regards to mundane senses (relevant super-senses can better discern where the actual source of the power is).

Insidious removes the sensory signature of the effects of a power from all senses, including the mundane sense of touch. The power's effects are considered to have Partial Concealment from all senses, generally resulting in a DC to detect of 10 + Stealth. If a power is both Subtle to all relevant senses and Insidious, it never downgrades a character's own Concealment; a fully Concealed character using Subtle, Insidious powers is exceedingly hard to detect for opponents who don't have either superior Perception skills or means of countering its concealment, even when slinging powers in the heat of battle.

Senses that Counter or Penetrate Concealment only fully counter natural concealment. Concealment from powers is instead reduced one step; Total to Partial, Partial to None. Resultantly, the ability to Counter Concealment does not automatically reveal opponents with concealing powers; a Perception check is still required. A character with the Hide in Plain Sight advantage treats natural concealment as concealment from a power for this purpose.

The Illusion power costs 1 PP per rank, and affects vision. Additional Sense is an Extra, which costs a flat 1 PP per rank of the Extra. Each rank allows the illusion to affect another sense. Two ranks affect an entire sense type.

The Counters Illusion Sense allows the character to immediately receive an Insight check upon perceiving an illusion to disbelieve it (the GM may roll this check in secret if desired). It also allows the character to substitute Will or Perception for Insight when attempting to disbelieve an illusion.